Active Record supports MySQL >= 5.0

Currently some features uses `information_schema` (e.g. foreign key
support). `information_schema` introduced since MySQL 5.0.

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -52,7 +52,6 @@ module ActiveRecord
       INDEX_TYPES  = [:fulltext, :spatial]
       INDEX_USINGS = [:btree, :hash]
 
-      # FIXME: Make the first parameter more similar for the two adapters
       def initialize(connection, logger, connection_options, config)
         super(connection, logger, config)
         @quoted_column_names, @quoted_table_names = {}, {}
@@ -65,6 +64,10 @@ module ActiveRecord
         else
           @prepared_statements = false
         end
+
+        if version < '5.0.0'
+          raise "Your version of MySQL (#{full_version.match(/^\d+\.\d+\.\d+/)[0]}) is too old. Active Record supports MySQL >= 5.0."
+        end
       end
 
       CHARSETS_OF_4BYTES_MAXLEN = ['utf8mb4', 'utf16', 'utf16le', 'utf32']
@@ -98,12 +101,8 @@ module ActiveRecord
         true
       end
 
-      # MySQL 4 technically support transaction isolation, but it is affected by a bug
-      # where the transaction level gets persisted for the whole session:
-      #
-      # http://bugs.mysql.com/bug.php?id=39170
       def supports_transaction_isolation?
-        version >= '5.0.0'
+        true
       end
 
       def supports_explain?
@@ -119,17 +118,15 @@ module ActiveRecord
       end
 
       def supports_views?
-        version >= '5.0.0'
+        true
       end
 
       def supports_datetime_with_precision?
         version >= '5.6.4'
       end
 
-      # 5.0.0 definitely supports it, possibly supported by earlier versions but
-      # not sure
       def supports_advisory_locks?
-        version >= '5.0.0'
+        true
       end
 
       def get_advisory_lock(lock_name, timeout = 0) # :nodoc:

activerecord/test/cases/defaults_test.rb

@@ -201,8 +201,7 @@ if current_adapter?(:Mysql2Adapter)
 
       assert_equal '0', klass.columns_hash['zero'].default
       assert !klass.columns_hash['zero'].null
-      # 0 in MySQL 4, nil in 5.
-      assert [0, nil].include?(klass.columns_hash['omit'].default)
+      assert_equal nil, klass.columns_hash['omit'].default
       assert !klass.columns_hash['omit'].null
 
       assert_raise(ActiveRecord::StatementInvalid) { klass.create! }

railties/lib/rails/generators/rails/app/templates/config/databases/jdbcmysql.yml

@@ -1,4 +1,4 @@
-# MySQL.  Versions 4.1 and 5.0 are recommended.
+# MySQL. Versions 5.0 and up are supported.
 #
 # Install the MySQL driver:
 #   gem install activerecord-jdbcmysql-adapter

railties/lib/rails/generators/rails/app/templates/config/databases/mysql.yml

@@ -1,4 +1,4 @@
-# MySQL.  Versions 5.0+ are recommended.
+# MySQL. Versions 5.0 and up are supported.
 #
 # Install the MySQL driver
 #   gem install mysql2